This invention relates to a floating offshore structure and more particularly to a floating platform used for the production and/or drilling of oil and gas.
Typically, in the oil industry, the offshore production and drilling for oil and gas has involved the use of a platform set on the ocean bottom and extending to a production or drilling platform above the water's surface. These types of operations are generally performed in water of less than 1300 feet. However, once drilling and/or production in deeper water began to be developed, the use of a solid structure stretching from the ocean surface to the bottom became impractical. Thus, alternative methods were developed for offshore drilling and production operations in deep water (over 1300 feet deep), and ultra deep water (over 2,000 feet deep).
Many different methods and devices have been proposed and used in deep water, most of which have involved some sort of floating platform. One such device is the tension leg platform, which is moored to the sea floor through the use of groups of vertically arranged high tension wires. Such arrangements, however, have not provided the control over the motion of the platform necessary for continuous, effective offshore operations. Specifically, the watch circle, defined as the circle of movement by the platform on the ocean's surface relative to the sea floor, may not be suitable for easily performing drilling and production operations. Additionally, the breakage of a high tension wire could have catastrophic effects on these operations, resulting in loss of life, platform, as well as threatening the environment.
Additional deep water offshore production and drilling apparatus include floating or semi-submersible platforms or vessels which are moored to the sea floor through the use of conventional catenary mooring lines. These types of platforms, however, while useful in deep water, can become problematic when used in ultra deep water because the vessel's watch circle can increase beyond acceptable levels when extremely lengthy catenary or other mooring lines are used. This is especially the case in high or rough seas, which can result in increased down time. Thus, such floating platforms are usually precluded from operating in ultra deep water.
One type of device that has been developed for use in deep and ultra deep water, and which claims to reduce the forces on the platform caused by the waves and other phenomena near the surface of the ocean is the cylindrical SPAR. An example of such a SPAR is disclosed in U.S. Pat. No. 4,702,321 to Horton. Such prior SPAR designs have been cylindrical in shape throughout their length. These types of floating cassions, however, have only been able to be used sparingly due to their expense and difficulty to manufacture. Not only must a cylindrical SPAR be fabricated at a specially designed facility, but they are very expensive to manufacture and, thus, only practical in unique situations where the anticipated production from the platform is very high. Also, the commission times for these SPARs can be very long.
Additionally, such prior art SPARs have had solid sides throughout their length and, thus, allow a substantial degree of movement both longitudinally and vertically, as well as in the pitch, roll, and yaw directions. This can cause an increased shutdown time for well production in times of bad weather or intense currents. Undersea currents can also create vortex-induced vibrations, which cause shaking of the entire structure due to the passing of undersea currents around the cylindrical platform. This also can cause safety concerns, as well as increased shutdown time. Additionally, the risers which bring oil up from the bottom of the ocean travel through the center of the prior art SPAR with no outlet to the sea other than that at the SPAR's bottom. Thus, if a breakage or leak occurs in the risers while in the middle of the SPAR body, such leaks have no way to escape and a dangerous situation can be created.